C?H/π‐Interaction‐Guided Self‐Assembly in π‐Conjugated Oligomers

We report CH/π hydrogen‐bond‐driven self‐assembly in π‐conjugated skeletons based on oligophenylenevinylenes (OPVs) and trace the origin of interactions at the molecular level by using single‐crystal structures. OPVs were designed with appropriate pendants in the aromatic core and varied by hydrocarbon or fluorocarbon tails along the molecular axis. The roles of aromatic π‐stack, van der Waals forces, fluorophobic effect and CH/π interactions were investigated on the theromotropic liquid crystallinity of OPV molecules. Single‐crystal structures of hydrocarbon OPVs provided direct evidence for the existence of CH/π interactions between the π‐ring (H‐bond acceptor) and alkyl C? H (H‐bond donor). The four important crystallographic parameters, d_c?x=3.79 Å, θ=21.49°, φ=150.25° and d_Hp?x=0.73 Å, matched in accordance with typical CH/π interactions. The CH/π interactions facilitate the close‐packing of mesogens in x–y planes, which were further protruded along the c axis producing a lamellar structure. In the absence of CH/π interactions, van der Waals interactions drove the assembly towards a Schlieren nematic texture. Fluorocarbon OPVs exhibited smectic liquid‐crystalline textures that further underwent Smectic A (SmA) to Smectic C (SmC) phase transitions with shrinkage up to 11 %. The orientation and translational ordering of mesogens in the liquid‐crystalline (LC) phases induced H‐ and J‐type molecular arrangements in fluorocarbon and hydrocarbon OPVs, respectively. Upon photoexcitation, the H‐ and J‐type molecular arrangements were found to emit a blue or yellowish/green colour. Time‐resolved fluorescence decay measurements confirmed longer lifetimes for H‐type smectic OPVs relative to that of loosely packed one‐dimensional nematic hydrocarbon‐tailed OPVs.     

Silicon‐Containing Formal 4π‐Electron Four‐Membered Ring Systems: Antiaromatic,Aromatic, or Nonaromatic?

Density functional theory calculations (B3LYP) have been carried out to investigate the 4π‐electron systems of 2,4‐disila‐1,3‐diphosphacyclobutadiene (compound 1 ) and the tetrasilacyclobutadiene dication (compound 2 ). The calculated nucleus‐independent chemical shift (NICS) values for these two compounds are negative, which indicates that the core rings of compounds 1 and 2 have a certain amount of aromaticity. However, deep electronic analysis reveals that neither of these two formal 4π‐electron four‐membered ring systems is aromatic. Compound 1 has very weak, almost negligible antiaromaticity, and the amidinate ligands attached to the Si atoms play an important role in stabilizing this conjugated 4π‐electron system. The monoanionic bidentate ligand interacts with the conjugated π system to cause π‐orbital splitting. This ligand‐induced π‐orbital splitting effect provides an opportunity to manipulate the gap between occupied and unoccupied π orbitals in conjugated systems. Conversely, compound 2 is nonaromatic because its core ring does not have a conjugated π ring system and does not fulfill the requirements of a Hückel system.     

Is Cyclopropane Really the σ‐Aromatic Paradigm?

Dewar proposed the σ‐aromaticity concept to explain the seemingly anomalous energetic and magnetic behavior of cyclopropane in 1979. While a detailed, but indirect energetic evaluation in 1986 raised doubts—“There is no need to involve ‘σ‐aromaticity’,”—other analyses, also indirect, resulted in wide‐ranging estimates of the σ‐aromatic stabilization energy. Moreover, the aromatic character of “in‐plane”, “double”, and cyclically delocalized σ‐electron systems now seems well established in many types of molecules. Nevertheless, the most recent analysis of the magnetic properties of cyclopropane (S. Pelloni, P. Lazzeretti, R. Zanasi, J. Phys. Chem. A 2007 , 111, 8163–8169) challenged the existence of an induced σ‐ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the σ‐aromatic stabilization of cyclopropane directly for the first time, fails to find evidence for a significant energetic effect. According to ab initio valence bond (VB) computations at the VBSCF/cc‐PVTZ level, the σ‐aromatic stabilization energy of cyclopropane is, at most, 3.5 kcal mol^?1 relative to propane, and is close to zero when n‐butane is used as reference. Trisilacyclopropane also has very little σ‐aromatic stabilization, compared to Si₃H₈ (6.3 kcal mol^?1) and Si₄H₁₀ (4.2 kcal mol^?1). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported.     

Can CH⋅⋅⋅π Interactions Be Used To Design Single‐Chain Magnets?

This theoretical study suggests that CH???π stacking interactions between monomeric units can be used to design novel single‐chain magnets (SCMs), as the sign of coupling is predictable and such chains inherently yield negative axial anisotropy, a condition often difficult to achieve in conventional SCMs.     

An Interplay of Cooperativity between Cation⋅⋅⋅π, Anion⋅⋅⋅π and C?H⋅⋅⋅Anion Interactions

Mixed cation (Li⁺, Na⁺ and K⁺) and anion (F^?, Cl^?, Br^?) complexes of the aromatic π‐surfaces (top and bottom) are studied by using dispersion‐corrected density functional theory. The selectivity of the aromatic surface to interact with a cation or an anion can be tuned and even reversed by the electron‐donating/electron‐accepting nature of the side groups. The presence of a methyl group in the ? OCH₃, ? SCH₃, ? OC₂H₅ in the side groups of the aromatic ring leads to further cooperative stabilization of the otherwise unstable/weakly stable anion???π complexes by bending of the side groups towards the anion to facilitate C? H???anion interactions. The cooperativity among the interactions is found to be as large as 100 kcal mol^?1 quantified by dissection of the three individual forces from the total interaction energy. The crystal structures of the fluoride binding tripodal and hexapodal ligands provide experimental evidence for such cooperative interactions.     

Does a Concerted Non‐Insertive Mechanism Prevail over a σ‐Insertive Mechanism in Catalytic Cyclohydroamination by Magnesium Tris(oxazolinyl)phenylborate Compounds? A Computational Study

The present study comprehensively explores alternative mechanistic pathways for intramolecular hydroamination of 2,2-dimethyl-4-penten-1-amine (1) by [{To(M)}MgMe] (To(M)=tris(4,4-dimethyl-2-oxazolinyl)phenylborate) (2) with the aid of density functional theory (DFT) calculations. A single-step amidoalkene → cycloamine conversion through a concerted proton transfer associated with N-C ring closure has been explored as one possible mechanism; its key features have been described. This non-insertive pathway evolves via a six-centre TS structure featuring activation of the olefin unit towards nucleophilic amido attack outside the immediate vicinity of the metal centre by amino proton delivery and describes a viable mechanistic variant for alkaline-earth metal-mediated aminoalkene hydroamination. However, herein is presented sound evidence for the operation of the Mg-N amido σ-bond insertive mechanism, its turnover-limiting activation barrier is found to be 5.0 kcal mol(-1) lower than for the non-insertive mechanism, for the cyclohydroamination of 2,2-disubstituted 4-aminoalkenes by a [{To(M)}Mg-NHR] catalyst. The operative mechanism involves rapid equilibria of the {To(M)}Mg-amidoalkene resting state 3 with its amine adduct, easily accessible and thermodynamically disfavoured, hence reversible, 1,2-olefin insertion into the Mg-N amido σ-bond with ring closure at 3, linked to turnover-limiting Mg-C azacycle tether aminolysis by an adduct substrate molecule, followed by facile cycloamine liberation to regenerate the active catalyst species 3. The following aspects are in support of this scenario: 1) the derived rate law is consistent with the experimentally obtained empirical rate law; 2) the reasonable agreement between the computationally estimated and the observed value of the primary KIE; 3) the assessed effective activation barrier for turnover-limiting aminolysis matches empirically determined Eyring parameters remarkably well; and 4) the observed resistance of isolated 3 to undergo amidoalkene cycloamine/cycloamido transformation until further quantities of substrate is added is consistently explained. The herein unveiled insights into the structure-reactivity relationships will undoubtedly govern the rational design of alkaline-earth metal-based catalysts and likely facilitate further advances in the area.     

Do Rhodium Bis(σ‐amine‐borane) Complexes Play a Role as Intermediates in Dehydrocoupling Reactions of Amine‐boranes?

The recently synthesized rhodium complex [Rh{P(C₅H₉)₂(η²‐C₅H₇)}(Me₂HNBH₃)₂]BAr^F₄ ( 2 ), which incorporates two amine‐boranes coordinated to the rhodium center with two different binding modes, namely η¹ and η², has been used to probe whether bis(σ‐amine‐borane) motifs are important in determining the general course of amine‐boranes dehydrocoupling reactions. DFT calculations have been carried out to explore mechanistic alternatives that ultimately lead to the formation of the amine‐borane cyclic dimer [BH₂NMe₂]₂ ( A ) by hydrogen elimination. Sequential concerted, on‐ or off‐metal, intramolecular dehydrogenations provide two coordinated amine‐borane molecules. Subsequent dimerization is likely to occur off the metal in solution. In spite of the computationally confirmed presence of a BH???NH hydrogen bond between amine‐borane ligands, neither a simple intermolecular route for dehydrocoupling of complex 2 is operating, nor seems [Rh{P(C₅H₉)₂(η²‐C₅H₇)} B ]⁺ to be important for the whole dehydrocoupling process.